A. Field of Invention
The present invention relates generally to driver circuits and more particularly to a new and improved circuit used to drive a solenoid coil.
B. Description of Related art
When operating a solenoid coil, for example as a valve actuator, power consumption can be a concern, particularly when high power consumption causes coil heating. When multiple solenoid actuated valves are grouped and required to have high speed operation capability, it is desirable to reduce power consumption while not reducing the voltage necessary for high speed coil excitation. Circuitry has been designed to cause the voltage directed to the coil to include an initial high voltage "spike" followed by a lower "hold" voltage. Such circuitry is designed to initiate rapid excitation of the solenoid with the voltage spike and maintain the excitation of the solenoid with the lower voltage, hold portion of the signal. In this way, the relatively high voltage necessary to quickly and efficiently operate a solenoid coil is not continued during the portion of the operation cycle when high voltage is not required to maintain the solenoid in operation.
FIG. 1 represents a prior art solenoid coil drive circuit that utilizes a spike and hold method of operating a solenoid. The spike and hold method activates the solenoid with a high voltage spike (usually two or three times the rated voltage of the coil) for only long enough to switch the position of the solenoid armature. The input voltage is reduced to a holding voltage (usually about one half the rated coil voltage) for the remainder of the on cycle. By using a lower voltage to hold the solenoid armature in the "on" position, less power is used when operating the solenoid. The wave forms in FIGS. 2a and 2b represent a common input signal and the resulting voltage across the solenoid coil generated by the conventional spike and hold circuit such as diagramed in FIG. 1. A square wave signal is applied to the control signal input terminals. When the control signal is high, a transistor (Q4 in FIG. 1) is turned on which allows current to flow through the solenoid coil. Another transistor (Q1 in FIG. 1) is coupled to the control signal input terminal through a capacitor (C2 in FIG. 1) and a resistor (R2 in FIG. 1) and is momentarily turned on at the rising edge of the control signal. This initiates a timer circuit to output a pulse of a duration that is determined by the combination of a resistor (R1 in FIG. 1) and a capacitor (C1 in FIG. 1). The output pulse from the timer circuit turns on two other transistors (Q2 and Q3 in FIG. 1) which allow voltage (V1) to be applied to the coil. Diode D1 isolates voltage V2 from V1 when V1 is applied to the coil. When the timer circuit output pulse terminates, transistors Q2 and Q3 turn off. This isolates voltage V1 from the solenoid coil and allows a second voltage (V2) to be applied to the coil. The voltage V2 represents the lower holding voltage and will continue to hold the solenoid armature in the "on" position until the control voltage is removed from the control signal terminals. This turns off transistor Q4 which prevents current from flowing though the coil.
The circuit in FIG. 1 requires at least two different voltage levels (V1, V2, and possible a third level for the timer) as well as a control signal input. Extensive circuitry is necessary to insure correct timing sequences between the timer and the transistor as well as to isolate the different voltage levels during activation of the solenoid coil.